(A) Field of the Invention
This invention relates to a process for retorting an organic oil-bearing solid, notably an oil shale, wherein the heat requirements of the retorting are supplied by the partial indirect heating of internally generated, or in situ combustion gases, and the combustion air, with the production of a by-product gas stream having a heating value high enough to be useful as a fuel.
(B) Description of the Prior Art
Many retorting processes, such as the traveling grate and gas combustion processes, provide heat for retorting organic oil-bearing solids, notably oil shales, by combustion of gas and/or organic residues (i.e., coke-like materials) which remain on the solids after pyrolysis.
Typically oil is recovered from such oil-bearing solids, or shales, by retorting the solids, or shales, on traveling, or circular, grate retorts (traveling grate retorts). Retorts within which such processes are conducted include, or are constituted of, a series of zones, typically a pyrolysis zone, an organic residue combustion zone, and cooling zones, typically one or a series of cooling zones. In such processes, e.g., a particulate coarse raw shale is laid down on a grate, and pyrolysis is carried out by contacting the shale with a hot non-reactible, non-oxygen containing flue gas which is generated by burning some of the low Btu product gas in an external combustor. The combustion gases are quenched with a part of the in situ generated product gas to moderate the temperature to a satisfactory level for pyrolysis. The hot flue gas is passed downwardly through the bed of shale, and grate, and the shale heated, while the flue gas is cooled. The flue gas, which carries the shale oil and water vapor, is removed from the pyrolysis zone and passed to a product recovery section located externally of the pyrolysis zone. In the organic residue combustion zone, the organic residue carried by the shale is burned to supply additional process heat. Air, usually diluted with recycle gas or steam to reduce oxygen concentration to moderate temperatures, is introduced into the organic residue combustion zone. The oxygen in the air combusts the organic residue on the retorted shale, this heating the gas. The gas, heated by combustion, flows downwardly, heating the unretorted shale to retorting temperature, the oil and gas released by pyrolysis being swept downwardly with the gas and cooled.
The gases from the pyrolysis and organic residue zones are cooled and processed through product recovery equipment to recover product shale oil. The separated low Btu gas is used as process fuel and dilution gas after a portion has been preheated.
Whereas such process has achieved some success, it is not generally possible to operate the retort to produce a product gas with adequate heating value for use as fuel in process heaters, unless rich shales are used as feeds. A major difficulty of this process is that the pyrolysis gas is comingled with combustion products and nitrogen from the combustion air. Thus, the heating value of the product gas depends upon the shale richness and the heat requirement for the process. The richness of the shale thus affects heating value through the amount of gas released by retorting. The richer the shale, the more pyrolysis gas is released. The heat requirements for the process affects the quality by setting the amount of oxygen needed to supply heat through combustion of gas and organic residue. The larger the heat requirements of the process, the more oxygen is needed. The more oxygen is needed the greater the consumption of pyrolysis gas, and the greater the dilution of pyrolysis gas with nitrogen and combustion products, this thereby reducing the heating value of the gas. Moisture content can add significantly to the heat requirements for the process. Thus, lean, wet shales can produce gas products with heating value unsatisfactorily low for many fuel uses, in some cases, as low as 40 Btu/SCF. For lean shales with high moisture content, the heating value of the products may be unsatisfactorily low for direct combustion in furnaces, boilers, process heaters and other combustion devices. Supplemental fuel to increase the gas heating valve to the level that it will burn with a stable flame may be necessary. The cost of supplemental fuel to promote combustion of the product can be prohibitively expensive.
Improvement of this process has been made to increase the heating value of the product gas by providing an indirect heating scheme to produce a hotter gas for injection into the pyrolysis zone without the addition of diluting nitrogen and combustion products. In such improvement, supplemental fuel for use in furnaces burning the product gas is not required. In accordance with this process, the first zone combustor is eliminated, and pyrolysis heat is supplied by heating a recycle stream externally in a furnace, at sufficiently high temperature to produce retorting temperatures in the bed. The heating value of the product gas in such process configuration is thus higher than in that previously described because heat requirements for the pyrolysis zone do not involve the introduction of air into the heat carrying gas, this reducing the dilution of evolved gas products in that zone. In such process, the gas from the first zone may be combined with gas from the second zone, or each may go through product recovery steps separately. In accordance with the latter, a portion of the gas is used as a heat carrier through heating in an external furnace, and another portion is used as furnace fuel.
Albeit, this latter process scheme considerably improves the heating value of the product gas, high temperature, large heat duty and complex design features for the preheat furnace combine to make this process less than satisfactory. The process is too costly, and the thermal efficiency of the process is lowered. Even the capacity of the pyrolysis unit appears lessened because the furnace outlet gas temperature (and thereby the retort gas inlet temperature) may be reduced due to temperature limitations of furnace materials; and hydrocarbons that are present in the gas may be thermally cracked.
There is a need, and large incentives exist, for means of improving the heating value of such product gases to levels whereby shales, especially lean, wet shales, can be retorted to produce by-product gasses with high enough heating value that they can be burned in combustion devices with minimal, and preferably without the use of supplemental fuel.